What is the best GPS receiver?
Do you mean best, or most popular?
Nearly all of the GPS receivers work reasonably well. Main differences come from speed to initial lock, ability to log where you've been, sensitivity to GPS signals, etc. Many users use the Rikaline and BU-303 models with great success. Here is a thread that compares recievers side by side:
GPS receiver tests by www.pocketgps.co.uk